A problem exists, for example, with quarter wave length antennas that are installed on vehicles, by reason of the fact that the surrounding metal structure has a pronounced capacitive effect which can drastically change the antenna's frequency from that of its uninstalled condition. With quarter wave length antennas, a quarter wave of each oscillation must occur in the transmission line or structure to which the antenna is electrically connected. In addition, metal structures close to the antenna produce a capacitive effect on the antenna to change its tuned frequency. A need therefore exists for a simple way of tuning the antenna after it is installed. In some instances the surrounding structures may have an effect on the tuning device itself, and for such environments a need exists for a tuning device that is substantially unaffected by its environment.
An object of the present invention therefore is the provision of a new and improved variable inductance tuning device which does not have internal inductance tuning structure which must be moved from a point outside of its housing.
A further object of the present invention is the provision of a new and improved impedance tuning device of the above described type having a primary inductance coil and an external electrically isolated tuning loop in which a parasitic electromagnetic field is produced which opposes the field of the primary inductance coil with a minimum capacitive effect and minimum resistance losses.
Another object of the invention is the provision of a tuning device for an antenna having a single tuning loop which when adjusted both changes the inductance of a loading coil for the antenna so that it has the correct electrical length to oscillate efficiently at a new frequency, and simultaneously changes a series tuned circuit so that it is tuned to pass the new frequency efficiently.
A further object of the present invention is the provision of a new and improved device of the above described type wherein the environment has substantially no effect on the device's adjustment of its transmitted frequency.
Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description of the preferred embodiments that are described with reference to the accompanying drawings.